The performance of many coatings such as paints are governed by the mechanical properties of one or more organic polymers which serve either as the coating per se or as a binder for other components of the coating, such as pigments and fillers. For use in automotive paints, it is desirable that such polymers be hard at ambient temperature, as illustrated, for example, by a Knoop hardness number (KHN) greater than about 5 MPa (L. Dillinger, "Hardness Testing", LECO Corp., 3000 Lakeview Ave., St. Joseph, Mich.; and ASTM D 1474-68). It is also desirable that such polymers retain a certain degree of stiffness and elasticity at use temperatures of 60.degree. C. or higher, for example, exhibit a Young's modulus (E) greater than 10 MPa Amorphous polymers exhibit such properties only when their glass transition temperature T.sub.g is equal to or greater than the uppermost use temperature. The present invention relates to a new class of coating formulations to achieve these objectives.
Coatings and films are commonly characterized as either brittle or ductile depending upon the manner in which they fail under tensile loads (I. M. Ward, Mechanical Properties of Solid Polymers, Chap. 12, John Wiley & Sons, London, 1971). Brittle failure occurs at relatively small strains, for example.ltoreq.20%, following a monotonic increase in the load. By contrast, ductile failure occurs at greater elongations, following a peak in the load/extension curve which is indicative of necking. Highly cross-linked, thermosetting polymer resins are generally brittle, often with elongations&lt;10%, whereas high-molecular-weight, linear thermoplastic polymers typically display a change in failure mode with temperature. At temperatures much below T.sub.g most thermoplastics are brittle, but they undergo a transition from brittle to ductile failure as the temperature is increased, and the temperature of this transition generally increases with increasing strain rate.
Ductility (especially elongations&gt;10%) is a desirable property for coatings on flexible substrates and also for coatings, such as automotive paints, on metals, because ductility contributes to the ability of the coating to survive impacts which dent or bend the substrate without causing cracking or peeling. The present invention provides a means to prepare coatings with a good balance of hardness, ductility, and stiffness, from very high-molecular-weight polymers, without the need for conventionally high amounts of volatile organic components and without the need for cure chemistry.
Aqueous colloidal dispersions of polymers are increasingly important in the paint industry because the coating constituents can be obtained in relatively concentrated form (&gt;20%), at moderate viscosities, and with little or no need for volatile organic solvents which constitute undesirable side-products in paint applications. However, the drying of such dispersions to form uniform, crack-free coatings are subject to certain well-known limitations. Such dispersions have been characterized, in each case, by a minimum film formation temperature, MFT, which is typically a few degrees below the glass transition temperature T.sub.g of the colloidal-polymer particles (See, for example, G. Allyn, Film Forming Compositions, R. R. Myers & J. S. Long, Ed., Marcel Dekker, N.Y., 1967). To whatever extent a polymer in a dispersion may be plasticized by other components of the dispersion, the MFT may be reduced accordingly. If a dispersion is dried at a temperature (T) less than the MFT, a multitude of microscopic cracks, so-called "mud-cracks", which destroy the integrity of the coating, tend to develop late in the drying process.
During drying, the actual temperature of an aqueous dispersion may be limited, by evaporative cooling, to a value much less than the temperature of the surrounding atmosphere. Thus, regardless of oven temperature, coating temperatures typically do not exceed about 35.degree. C. under atmospheric pressure, until substantially all of the water has been lost (F. Dobler et al., J. Coil. Poly. Sci., 152, 12 (1992)). This means that dispersions with MFT&gt;35.degree. C. are not generally useful for coating applications. But polymers with unplasticized MFT.ltoreq.35.degree. C. generally do not provide adequate mechanical properties, including hardness at higher temperatures of use.
Two strategies have most commonly been employed to bridge this gap between requirements for film formation and requirements for hard coatings with elevated temperatures of use. First, volatile organic plasticizers (often described as coalescing aids or film formers) have been added to the dispersion. These dissolve in the polymer and lower its T.sub.g during drying, but ultimately volatilize at a later stage of drying, leaving the final resin at a higher T.sub.g. This strategy, however, conflicts with economic and environmental motivations to limit the amount of volatile organic content (VOC's) in coating formulations.
A second strategy has been to formulate the dispersion with a low-molecular-weight thermosetting resin which, prior to cure, has sufficiently low T.sub.g to provide film formation. After drying, curing at elevated temperatures, which result in cross-linking and chain extension reactions, raise the T.sub.g and establish the ultimately desired mechanical properties.
In recent years, it has been found that coatings prepared from blends of film-forming and non-film-forming aqueous dispersions of polymer colloids can be prepared with little or no need of a coalescing aid. Friel (EP 0 466 409 A1, Apr. 7, 1991) has shown that dispersion blends of a "soft emulsion polymer" with T.sub.g &lt;20.degree. C., at 20 to 60% by weight, in combination with a second "hard emulsion polymer" with T.sub.g &gt;20.degree. C. exhibit MFT's.ltoreq.9.35.degree. C., without the need for a coalescing aid. All cited examples, however, exhibited hardnesses of KHN.ltoreq.2.7 MPa.
Snyder (U.S. Pat. No. 5,344,675, Sep. 6, 1994; U.S. Pat. No. 5,308,890, May 3, 1994) has shown that blends of a film-forming dispersion of a "multi-stage" latex polymer and a second non-film-forming dispersion can be used to form coatings without the need for a coalescing aid. Snyder specifies that each latex particle in the "multi-stage" component must contain between 50 and 95% by weight of a polymer with T.sub.g &lt;50.degree. C. and a second polymer of higher T.sub.g. A certain balance was achieved between coating hardness, impact resistance and flexibility. The '890 patent also states at column 3, lines 25-30, that "a comparable balance of these properties cannot be obtained by the use of other types of systems, such as, for example, a random copolymer, simple blends of conventional emulsion polymers, a single type of multi-stage polymer, and the like." Among the examples cited for coatings made without a coalescing aid, three have a KHN&gt;5.5 MPa, but these are characterized by a reverse impact resistance&lt;2 inch-pounds and a flexibility corresponding to mandrel flexibility of 1/2". (According to ASTM D 1737-62, a mandrel bend of 1/2" is approximately equivalent to a tensile elongation of 6.8%). Further examples are cited with a KHN between 4.0 and 5.5 MPa, but with an impact resistances ranging from 4 to 60 inch-pounds and a mandrel flexibility of 1/2" to 3/16".